(1) Field of the Invention
The present invention relates to an acceleration sensor that detects acceleration with an inertia mass, for a seat belt retractor, and a seat belt retractor including the acceleration sensor.
(2) Description of Related Art
A vehicle such as an automobile is equipped with a seat belt device including a retractor, for protecting an occupant sitting in a seat. The occupant pulls out a webbing (a seat belt) from the retractor and wears the webbing. When an acceleration sensor of the retractor detects an acceleration of a predetermined value or greater caused by collision of the vehicle or the like, the acceleration sensor activates a locking mechanism of the retractor. The locking mechanism stops the webbing from being pulled out, and the webbing restrains the occupant on the seat. Further, the acceleration sensor also detects acceleration caused by tilting of the vehicle, and activates the locking mechanism. That is, the acceleration sensor detects also tilting of the vehicle in a rollover accident, and activates the locking mechanism.
In general, an acceleration sensor includes an inertia mass that is displaced by an inertia force due to acceleration, and detects acceleration with the inertia mass. Further, a conventionally known acceleration sensor includes a ball being an inertia mass, and a pole (a sensor lever) rotatably supported by a pair of arms (supports) of a bracket (a sensor holder) (see Japanese Utility Model Application Publication No. 63-114867).
In the conventional acceleration sensor disclosed in Japanese Utility Model Application Publication No. 63-114867, the ball is displaced by an inertia force, causing the sensor lever to rotate to be displaced. The displaced sensor lever of the acceleration sensor activated in this manner activates the locking mechanism of the retractor.
When the conventional acceleration sensor is assembled, the interval between the pair of supports is widened, and a pin (a shaft) of the sensor lever is inserted into support holes of the pair of supports. At this time, the pair of supports must elastically deform. Further, taking into account of ease in mounting the sensor lever or movability of the mounted sensor lever, a certain clearance in the rotation axis direction must be provided between the sensor lever and the pair of supports. Still further, the components are different from each other in dimension, and there is play between the components. These conditions make it difficult to restrict shifting of the sensor lever in the rotation axis direction, and may result in deviation of the sensor lever from the intended position.
In particular, when the vehicle tilts, the sensor lever tends to shift in the rotation axis direction because its rotation axis tilts relative to the horizontal direction. Inappropriate positioning of the sensor lever may disadvantageously influence the stable operation of the acceleration sensor. Accordingly, there is a need to design the locking mechanism taking into account of positional deviation of the sensor lever, which hinders a reduction in size of the locking mechanism.